What book is it?
To make reading easier in the Bass(F)clef for bassists we transpose everything up an octave,so that LOOOOWWW G,is really the lowest G on your bass,the higher one is the octave,normal bass charts that's more or less assumed,unless noted otherwise.
If I understand your first question correctly,that's the melody with a harmony note under it.You can play both ,it's great practice,the principal note of the melody(commonly)would be on top.